1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus for checking the straightness of fuel rod tubes during several stages of tube manufacture.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods.
The tubes of the fuel rods and guide thimbles are contained within a typical fuel assembly in a closely packed aray in which the tubes are precisely spaced from each other. Optimum performance of the reactor core depends upon uniformity of the gaps between the tubes. And, thus in the manufacture of these tubes, much attention is devoted to ensuring that they are as straight as possible.
During tube manufacture, the current practice has been to perform straightening of all tubes by using a machine after final tube reduction and annealing stages but without previously determining their straightness. The straightness is checked immediately thereafter on a sampling basis by rolling the sampled tube across a table surface and using a feeler gauge to measure the maximum gap that occurs between the tube and the table surface. It is normally required that the gap not exceed 0.010 inch in a span of twelve inches of tube length. A second inspection of tube straightness is carried out for all tubes just prior to packing.
An important disadvantage of the current practice is that tube straightness is not checked until after completion of the stages of manufacture of the sampled tube. It would be most beneficial to know straightness at several stages of tube manufacture. First, a check of tubes entering the pilgering operation (i.e. elongation of a tube to a finished size over a stationary mandrel through effecting a reduction in both the diameter and wall thickness of the tube by means of two grooved dies which embrace the tube from above and below and roll in a constant cycle back and forth along the tube) would detect hooked tubes and prevent costly "wrecks" currently experienced from time to time. Additionally, an in-process check for straightness during the final pilgering operation would be useful to verify that the pilger die and mandrel relationships are correct. Next, a straightness check after final annealing would avoid the hazards of unnecessary straightening (end damage, cold work and crushing) and contribute to improved efficiency and yield. Finally, it would be useful to check tubes just prior to carrying out the standard ultrasonic test where out-of-straightness conditions are accentuated by the high rotational speeds and hamper the accuracy and reliability of the inspection process.
Consequently, a need exists for an alternative approach to tube straightness checking which facilitates realization of the benefits outlined above in a simple, low-cost and reliable manner.